"When Let’s Encrypt, a free certificate authority, started issuing 90 day TLS certificates for websites, it was considered a bold move that helped push the ecosystem towards shorter certificate life times. Beforehand, certificate authorities normally issued certificate lifetimes lasting a year or more. With 4.0, Certbot is now supporting Let’s Encrypt’s new capability for six day certificates through ACME profiles and dynamic renewal at:

- 1/3rd of lifetime left

- 1/2 of lifetime left, if the lifetime is shorter than 10 days"